Brown is calling on Trump to secure agreements from Mexico and Canada that "Buy American" provisions won't be weakened in negotiations -- and that Mexico, in particular, will enforce labor and environmental standards, rather than allowing lax conditions that have led companies to ship jobs there to continue.

"I'm sending him a letter asking him to do four things in the negotiations," he told CNN's Alisyn Camerota Monday on "New Day." "All four really matter for a better trade agreement among Canada, Mexico and the United States."

"The first is get up front commitments on 'Buy America,' and anti-outsourcing provisions. Second is make sure you don't play one American worker against another meaning farmers against manufactures," Brown added.

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"That's important that the President mediate that in these negotiations."

The vocal fair-trader is also urging Trump to develop sector-by-sector negotiating plans to avoid horse-trading one industry's protections for another's.

Brown, a member of the Senate Finance Committee, which has jurisdiction over trade deals, has been in close contact with Trump's White House on the topic.

More recently, Brown discussed NAFTA's renegotiation with Commerce Secretary Wilbur Ross last week, and has also spoken with Robert Lighthizer, Trump's nominee for US trade representative.

"American jobs shouldn't be up for negotiation and American workers can't be traded away as bargaining chips," Brown said in a statement accompanying his letter. "By setting high standards, putting workers ahead of corporations and refusing to compromise on outsourcing, we can create the best possible deal for all American workers."

Brown has reason to work with Trump beyond their shared skepticism about trade deals: He is up for re-election in the 2018 midterms in a state where Trump defeated Hillary Clinton by 9 percentage points. That means he'll need to outperform the Democratic presidential nominee with white working-class voters in order to win a third term.

Brown also wants a renegotiated NAFTA to abolish a mechanism that allows companies to ask an independent, international arbiter to decide whether countries' laws and regulations meet that country's free trade commitments.

And Brown wants US trade proposals to be made public both before and after negotiating rounds, increasing transparency as the deal is hashed out.

Before Trump's inauguration, Brown sent him a letter calling on him to abandon the Trans-Pacific Partnership -- a massive trade deal involving 12 Pacific Rim nations that had been a second-term priority for former President Barack Obama. Trump responded with a hand-written note thanking Brown for the "great letter" and saying, "I will never let our workers down."

"As I wrote to you right after your election, many candidates have promised big changes in US trade policy only to cave to corporate pressure upon reaching the White House," Brown wrote in Monday's letter to Trump.

He added, "By rethinking the US approach to negotiations and obtaining pre-negotiation commitments from our trading partners as outlined above, you can fulfill your campaign promises, set a new standard for a trade policy, and ensure our trade agreements benefit US workers and communities."